Physical Geography

Alaska's large size is matched by its great variety of surface features. Extending east-west across the state in rough bands are four regions: the North, or Arctic, Slope; the Brooks Range; the Central Uplands and Plains; and the Pacific Mountains.

The North Slope begins at the Arctic Ocean as an almost featureless plain. It is marked by shallow river valleys, sparse tundra vegetation, and numerous ponds, swamps, and marshes. Toward the south it becomes more rolling, especially at the edge of the Brooks Range.

The Brooks Range, a continuation of the Rocky Mountains, rises abruptly above the North Slope as a wall of peaks reaching elevations of 4,000 to 9,000 feet (1,200 to 2,700 m). This mountainous region includes several smaller ranges, notably the De Long, Baird, Endicott, and Philip Smith mountains. The northern limit of tree growth follows a zigzag pattern through the Brooks Range.

The Central Uplands and Plains region lies between the Brooks Range and the Pacific Mountains along the Gulf of Alaska. It is a vast expanse of alluvial lowlands and eroded, mountainous plateaus. The heavily eroded Seward, Koyukuk, Yukon, and Kuskokwim uplands overlook the broad, flat plains of the Yukon River system. Valleys are wooded but many of the uplands are barren. There are also areas that resemble the frost-scarred North Slope. In the Bering Sea lie two of the state's largest islands—Nunivak and St. Lawrence.

The Pacific Mountains consist of several major ranges that extend in a wide arc from the Aleutian Islands through the Panhandle, the narrow strip of Alaska between Canada and the ocean. These mountains are marked by jagged ridges, glaciers, deep valleys, and fjords. Snow-covered peaks jut out high above the lower, forested slopes.

The Aleutian Range, dotted by several active volcanoes, runs the length of the Alaska Peninsula. It merges on the northeast with the massive Alaska Range, site of 20,320-foot (6,194-m) Mount McKinley, the loftiest peak in North America. Rugged, towering peaks also mark the Chugach, Wrangell, and St. Elias mountains. Mount St. Elias, for example, soars to 18,008 feet (5,489 m) on the Alaskan-Canadian border. Glaciers flank many of the peaks; some, such as Bering, Guyot, Malaspina, and Muir glaciers, descend all the way to the sea.

In several areas the ranges of the Pacific Mountains are separated by lowlands, part of a series of troughs extending as far south as Washington and Oregon. These lowlands reach their greatest size in the basins around Anchorage. Islands and island groups include the Aleutians, the Alexander Archipelago, and Kodiak.

Hundreds of streams and rivers drain the state, but there are few large lakes. The chief river system is the Yukon, which flows into the Bering Sea. With its main tributaries, the Tanana and Koyukuk rivers, the Yukon drains most of central Alaska. The Kuskokwim River system, which flows through the southwest, also drains into the Bering Sea. In the north are the Colville and Noatak rivers; in the south, the Copper and Susitna rivers.

Largest lakes are Iliamna and Becharof, in the southwest, and Teshekpuk, on the North Slope. Small, deep lakes occur in mountainous areas, particularly in the southwest. Other small, but shallow, lakes are found in the alluvial lowlands along the Arctic and Bering Sea coasts.

Chief influences on the climate are Alaska's northerly latitude, its large land mass, and its coastal waters. Except along the southeast coast, winters are long and bitterly cold. Summers throughout the state are short and cool. Precipitation, both rain and snow, varies from large amounts in the Panhandle to negligible amounts along the Arctic shore.

The Panhandle has a relatively mild climate because of the moderating effects of offshore ocean currents and winds. Average temperatures at Juneau, for example, vary from about 25° F. (-4° C.) in January to about 55° F. (13° C.) in July. Sitka's average January temperature is slightly above freezing. Precipitation for the year is close to 90 inches (2,290 mm) in most areas, but some receive as little as 25 inches (635 mm) and others more than 150 inches (3,810 mm). Snowfall amounts to about 100 inches (2,540 mm) a year.

The western coast, from the Aleutians to the Seward Peninsula, is cooler and drier than the Gulf of Alaska coast. At Nome, temperatures average about 4° F. (-16° C.) in January and 49° F. (9° C.) in July. Average yearly precipitation is about 20 inches (510 mm), including as much as 80 inches (2,030 mm) of snow. Cold, moist winds, frequent fog, and sudden storms often make this region unpleasant.

Temperature ranges increase and precipitation decreases away from the coasts. January temperatures average well below 0° F. (-18° C.). July temperatures generally are 60° to 70° F. (16° to 21° C.), though readings of 90° F. (32° C.) or more occasionally occur. Yearly precipitation is less than 15 inches (380 mm), including an average snowfall of 40 to 60 inches (1,020 to 1,520 mm).

On the North Slope, winters are severe, summers are cool, and freezes may come in the warmest months. Arctic waters have a slight modifying effect. Barrow averages -16° F. (-27° C.) in January and 40° F. (4° C.) in July. Precipitation is less than elsewhere in Alaska, seldom exceeding 8 inches (200 mm) a year. Snowfall averages about 28 inches (710 mm) annually.

Forests cover about one-third of the state. They vary from tall, dense coniferous (evergreen) forests in the south and southeast to the woodlands of mixed coniferous and deciduous trees in the interior. The southern and southeastern forests are dominated by Sitka spruce and western hemlock, occasionally mixed with red cedar, Alaska cedar, and some red alder and cottonwood. These forests rapidly diminish north of the Alaska Range. The interior woodlands are variously composed of white and black spruce, Alaska birch, cottonwood, balsam, and aspen. Farther north the trees become increasingly sparse and stunted.

Much of the remaining land, especially in the interior and on the North Slope, is carpeted with low-growing tundra vegetation—grasses, mosses, and lichens—during the brief warm months. Most of the tundra region is underlaid by permanently frozen subsoil, called permafrost.